Hard Sci-fi author Charlie Stross wrote an essay earlier this year opining that its so unlikely a proposition that we might as well consider it an impossibility (Hey! like life spontaneously...oh, never mind).

What say the members of the Observatory? It's a fascinating topic to me...

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This is not to say that interstellar travel is impossible; quite the contrary. But to do so effectively you need either (a) outrageous amounts of cheap energy, or (b) highly efficient robot probes, or (c) a magic wand. And in the absence of (c) you're not going to get any news back from the other end in less than decades. Even if (a) is achievable, or by means of (b) we can send self-replicating factories and have them turn distant solar systems into hives of industry, and more speculatively find some way to transmit human beings there, they are going to have zero net economic impact on our circumstances (except insofar as sending them out costs us money).

He then postulates a one way one person trip to Proxima Centauri.

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We're sending them on a one-way trip, so a 42 year flight time isn't unreasonable. (Their job is to supervise the machinery as it unpacks itself and begins to brew up a bunch of new colonists using an artificial uterus. Okay?) This means they need to achieve a mean cruise speed of 10% of the speed of light. They then need to decelerate at the other end. At 10% of c relativistic effects are minor — there's going to be time dilation, but it'll be on the order of hours or days over the duration of the 42-year voyage. So we need to accelerate our astronaut to 30,000,000 metres per second, and decelerate them at the other end. Cheating and using Newton's laws of motion, the kinetic energy acquired by acceleration is 9 x 1017 Joules, so we can call it 2 x 1018 Joules in round numbers for the entire trip. NB: This assumes that the propulsion system in use is 100% efficient at converting energy into momentum, that there are no losses from friction with the interstellar medium, and that the propulsion source is external — that is, there's no need to take reaction mass along en route. So this is a lower bound on the energy cost of transporting our Mercury-capsule sized expedition to Proxima Centauri in less than a lifetime.

To put this figure in perspective, the total conversion of one kilogram of mass into energy yields 9 x 1016 Joules. (Which one of my sources informs me, is about equivalent to 21.6 megatons in thermonuclear explosive yield). So we require the equivalent energy output to 400 megatons of nuclear armageddon in order to move a capsule of about the gross weight of a fully loaded Volvo V70 automobile to Proxima Centauri in less than a human lifetime. That's the same as the yield of the entire US Minuteman III ICBM force.

For a less explosive reference point, our entire planetary economy runs on roughly 4 terawatts of electricity (4 x 1012 watts). So it would take our total planetary electricity production for a period of half a million seconds — roughly 5 days — to supply the necessary va-va-voom.

But to bring this back to earth with a bump, let me just remind you that this probe is so implausibly efficient that it's veering back into "magic wand" territory. I've tap-danced past a 100% efficient power transmission system capable of operating across interstellar distances with pinpoint precision and no conversion losses, and that allows the spacecraft on the receiving end to convert power directly into momentum. This is not exactly like any power transmission system that anyone's built to this date, and I'm not sure I can see where it's coming from.

Another problem:

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when we start examining the prospects for interplanetary colonization things turn gloomy again.

Bluntly, we're not going to get there by rocket ship.

Optimistic projects suggest that it should be possible, with the low cost rockets currently under development, to maintain a Lunar presence for a transportation cost of roughly $15,000 per kilogram. Some extreme projections suggest that if the cost can be cut to roughly triple the cost of fuel and oxidizer (meaning, the spacecraft concerned will be both largely reusable and very cheap) then we might even get as low as $165/kilogram to the lunar surface. At that price, sending a 100Kg astronaut to Moon Base One looks as if it ought to cost not much more than a first-class return air fare from the UK to New Zealand ... except that such a price estimate is hogwash. We primates have certain failure modes, and one of them that must not be underestimated is our tendency to irreversibly malfunction when exposed to climactic extremes of temperature, pressure, and partial pressure of oxygen. While the amount of oxygen, water, and food a human consumes per day doesn't sound all that serious — it probably totals roughly ten kilograms, if you economize and recycle the washing-up water — the amount of parasitic weight you need to keep the monkey from blowing out is measured in tons. A Russian Orlan-M space suit (which, some would say, is better than anything NASA has come up with over the years — take heed of the pre-breathe time requirements!) weighs 112 kilograms, which pretty much puts a floor on our infrastructure requirements. An actual habitat would need to mass a whole lot more. Even at $165/kilogram, that's going to add up to a very hefty excess baggage charge on that notional first class air fare to New Zealand — and I think the $165/kg figure is in any case highly unrealistic; even the authors of the article I cited thought $2000/kg was a bit more reasonable.

Whichever way you cut it, sending a single tourist to the moon is going to cost not less than $50,000 — and a more realistic figure, for a mature reusable, cheap, rocket-based lunar transport cycle is more like $1M. And that's before you factor in the price of bringing them back ...--Let me repeat myself: we are not going there with rockets. At least, not the conventional kind — and while there may be a role for nuclear propulsion in deep space, in general there's a trade-off between instantaneous thrust and efficiency; the more efficient your motor, the lower the actual thrust it provides. Some technologies such as the variable specific impulse magnetoplasma rocket show a good degree of flexibility, but in general they're not suitable for getting us from Earth's surface into orbit — they're only useful for trucking things around from low earth orbit on out.

For a less explosive reference point, our entire planetary economy runs on roughly 4 terawatts of electricity (4 x 1012 watts). So it would take our total planetary electricity production for a period of half a million seconds — roughly 5 days — to supply the necessary va-va-voom.

Given the trip is bound to take 42 years a more reasonable way of presenting things would be to say that a power generator of 1.3 GW would be needed if the output was released equally over the whole trip. That suddenly doesn't look as impossible anymore as there are nuclear centrals providing that kind of output. Considering losses & shorter acceleration/deceleration phases would require even more power but it's definitely an option, costly though it may be.

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Whichever way you cut it, sending a single tourist to the moon is going to cost not less than $50,000 — and a more realistic figure, for a mature reusable, cheap, rocket-based lunar transport cycle is more like $1M. And that's before you factor in the price of bringing them back ...

Given that it costs $1M for space tourists to pass a few days in the ISS & that there probably is a good profit margin included in that price, the cost to send someone to the moon with current solutions would probably be around those $1M. However if you're talking about low cost rockets that could divide the price by factors of ten or more that $1M seems quite overestimated.

Part of the issue is that thats not interplanetary, but interstellar colonization. We have a bit of a less distance to say, Mars our closest planetary neighbor that isn't quite actively hostile.

Interstellar is bound to be a one-way trip, with timelag and difficulties inherent to the problem. In the end though, the usual science fiction solutions present themselves.

Cryogenic Hibernation Pods will shorten the trip for the crew. An on/off crew rotation might make it even reasonable.

Realistic time frames. To mars: 2-3 months. To Jupiter: 3-4 years. 42 years seems like a tremendously short time, to the point that I'm skeptical of even that span. Of course, the proposed nearly magical ship travels with an external energy source. A more standard ship with an internal fuel storage would take far longer. The gulf of 60-100 years between the stars would be a distance not entirely uncrossable. It just won't be the same as crossing the globe.

The thing you have to remember is that there was a time that the ability to travel between the continents was not exactly something people were so capable of doing. If you reframe the proposition, it isn't so bad. You can't travel back and forth easily. You won't be taking multiple trips in a lifetime. You take it as a colonization, a one way ticket. There was a time where the world wasn't so small, and accessible.

Buzzard Ramjets, Laser /Sail Propulsion, there are plenty of fantasy propulsion systems that we've dreamed up. Plenty more that I'm sure we will dream up. Interplanetary and interstellar colonization isn't something that is an option. Carl Sagan once said something along the line that we shouldn't keep our eggs in one basket. I think we will come up with a way.

Once we can cheaply get into space and the moon, traveling elsewhere will become possible. It still won't be cost effective, but we should be able to have Martian colonies easily enough. I'm assuming we will be able to find some resource that makes it worth going, eventually. Interstellar travel is more complex as Stross laid out, but I have confidence that we will come up with at least one or two "magical" technologies that makes it at least possible in the next few decades.

If we can come up with a propulsion method that can accelerate at 1g for the entire trip, it won't really take that long to get to a lot of the nearby systems. Sure it will probably be an effectively one way trip, but just like there aren't a shortage of astronauts, I don't think there will be a shortage of volunteers.

Another problem not yet mentioned is radiation. It's harsh out there above the Van Allen belts. A three year trip to Mars (one year on surface, much weaker magnetic field) is equivalent to a lifetime dose:

I think this guy knows just enough science to write science fiction but not enough to write actual science.

A one person trip? Absurd. What if the one person suddenly died? Or went crazy? Fuck Proxima Centauri, lets go to that star over there! Biological/psych testing can account for everything. The infrastructure required to support the first human is huge, but to support that second, third, forth, etc goes down dramatically. So you either go totally robotic or a group of people.

As far as speed, well you constantly accelerate to the halfway point, turn around, then constantly decelerate the rest of the way. The only limit to this is how much acceleration the ship and cargo can handle and the mass of the propulsion system in relation to the force it can output. Given technological advances the past century I'm thinking its definitely possible to have such technology by the end of this century.

All of his prices are absurd. They assume we are launching this thing today. Why not make the assumption that we have multiple space elevators and a huge orbiting industrial complex to build things?

Lastly, we would never even consider doing this until we could do it on a massive scale. Sending one ship to one star is pointless. The chances of it arriving are already small, even over engingeering can only improve things so much. Instead, you spam probe. A dozen ships get sent to every nearby star that has a habitable planet. Ideally we'd send unmanned seeder probes ahead of time. These could travel much faster given they could have a greater (de)accelerate without squishies onboard. Have them dump biological payloads on near Earth-like planets for future colonization, then hang out in the system gathering data.

Given the trip is bound to take 42 years a more reasonable way of presenting things would be to say that a power generator of 1.3 GW would be needed if the output was released equally over the whole trip. That suddenly doesn't look as impossible anymore as there are nuclear centrals providing that kind of output. Considering losses & shorter acceleration/deceleration phases would require even more power but it's definitely an option, costly though it may be.

Given that his energy calculations are for an entirely off-board propulsion system moving only 2000 kg (Manned or robotic, that's a small craft), it really doesn't make it that much more possible. He's already at the edge of 'magic wand' technology.

Imagine aiming a laser to hit a moving target from a moving platform at a range of 2 light-years. You have no margin for error. Anything causing the craft to drift off course or receive less than constant power would be entirely catastrophic as there will be effectively no feedback from the target after a small fraction of the trip. Building in a reasonable safety net requires more energy.

Putting the engine and/or fuel on-board raises the energy requirements by several powers of 10.

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I think this guy knows just enough science to write science fiction but not enough to write actual science.

Originally posted by Peldor:Given that his energy calculations are for an entirely off-board propulsion system moving only 2000 kg (Manned or robotic, that's a small craft), it really doesn't make it that much more possible. He's already at the edge of 'magic wand' technology.

Imagine aiming a laser to hit a moving target from a moving platform at a range of 2 light-years. You have no margin for error. Anything causing the craft to drift off course or receive less than constant power would be entirely catastrophic as there will be effectively no feedback from the target after a small fraction of the trip. Building in a reasonable safety net requires more energy.

Putting the engine and/or fuel on-board raises the energy requirements by several powers of 10.

Make an army of self-replicating nano-bots. Land them on the moon. Have them convert the surface to solar cells. Use that energy to produce anti-matter. Use that as your fuel.

Still impossible. Even if we hand-wave away production difficulties (only 2 billion years for CERN to make a gram of anti-hydrogen), you can't contain enough antimatter to produce that much power. Electrically charged antiparticles are too repulsive to amass more than a tiny amount and electrically neutral antimatter is worse because you can't even use a magnetic bottle.

Putting the engine and/or fuel on-board raises the energy requirements by several powers of 10.

This would not be true if the fuel is antimatter. To get the energy indicated, 5 kilograms of antimatter would suffice (along with an equal mix of 5 kg of matter) assuming 100% efficiency. Using 1% efficiency, it would consume a total of 1,000 kg mass, or half your weight budget. Compared to modern rockets that only go to LEO, that's phenomenally better.

Producing and storing antimatter in quantities even close to that large represents an *enormous* engineering challenge, one that is well out of reach at present IMHO. However, it is not outside of the realm of scientific possibility.

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Another problem not yet mentioned is radiation. It's harsh out there above the Van Allen belts. A three year trip to Mars (one year on surface, much weaker magnetic field) is equivalent to a lifetime dose:

This to me is the bigger issue, for both the interstellar (far future) idea and the interplanetary (near future) idea. In the near future, radiation shielding is quite bulky to be effective. In the far future, you are adding additional radiation from .1C impacts with the (admittedly thin) interstellar medium. It's a tough problem.

Still impossible. You can't contain enough antimatter to produce that much power. Electrically charged anti-particles are too repulsive to amass more than a handful and electrically neutral antimatter can't be contained at all.

Store them outside the ship, contained in a magnetic field. There's plenty of room to keep them from getting crowded (it is *space* after all ), and we already must assume a way to scatter the thin interstellar medium away from the ship (to prevent radiation) so it's not a stretch to keep it away from the diffuse cloud of antimatter you trail as well.

Although there are certainly physical limits as to what the time frame of an interstellar trip would be, there are other factors that are perhaps far less limited. One often makes the assumption that the trip must last significantly less than a human lifetime, but exactly how long IS a human lifetime? If we make the assumption that we'll have access to anti-matter generators or space-medium jet engines, why not make the other reasonable assumption that humans could end up having much longer lifespans, on the order of hundreds of years, or more? How do the numbers change if we are allowed a 300 to 600 year travel time?

Indeed the human body is just another machine, albeit an enormously complex one, so there is no physical reason why it could not be maintained in near-perfect shape indefinitely, or why eventually consciousness could not be downloaded into a fully artificial body, allowing the very probes we could be using to scout to be the first "human" colonists.

Still impossible. You can't contain enough antimatter to produce that much power. Electrically charged anti-particles are too repulsive to amass more than a handful and electrically neutral antimatter can't be contained at all.

Create a piece of anti-iron (fusion of anti-particles also releases energy, so we're still OK.). Now, magnetize the anti-iron, and keep it in a magnetic bottle.

Or simply make a big chunk of neural antimatter, spit some positrons at it to give it a nice charge, and push it in front of you as you travel with an electric force. No biggie.

Most antimatter problems have more to do with engineering than physics.

Originally posted by Pakkal:Although there are certainly physical limits as to what the time frame of an interstellar trip would be, there are other factors that are perhaps far less limited. One often makes the assumption that the trip must last significantly less than a human lifetime, but exactly how long IS a human lifetime?

The trip doesn't have to be less, it just can be. Remember, two years at 1 g acceleration and you too can be anywhere in the universe.

That's a really interesting article. It's basically true and is an issue I have long pondered.

Acceleration\deceleration is the real problem. I mean even if you can accelerate to even a % of the speed of light, it is possible that mass can be lost on the outer surface of the ship.

The only possibility is to use a large electromagnetic source that propels a field emitted around the ship.

Plus with trying to fly that far and at hyper-speeds you can't hit anything - even a small object - as that would be a helluva lot of momentum.

Though by using the right wavelength of energy it wold be possible to saturate the molecules of he "transported" including people, food, supplies, etc.

Then as acceleration occurs the amount of energy required to break bonds goes up exponentially allowing the ship to maintain structurally integrity during acc\dec.

Of course a type of PID control would have to be enacted in order to verify that different distances acc\dec at the right point and with the right amount of energy.

This also requires an initial long trip to setup the receiver end of the field generator. But it could be possible to have the energy field dissipate based on the distance traveled, mass of object and loss due to friction.

Originally posted by htom:Possible, through sufficently advanced technology that would appear to us as magic.

Has any other author, nevermind sci-fi author, had a quote that has been used more times?

Who knows what we will have discovered a thousand years from now. Five thousand. Things that are absolutely impossible from our perspective may very well be nothing to the humans of the year 8000. Reactionless drives! Who says you can't accelerate to 99% of the speed of light? Particle collisions at that speed? Well, of course they will be able to generate a field to protect the ship, and a side effect will be that the collision with the field will generate energy that the ship can absorb and store.

I wish I could see what everything will be like then. Hell, maybe we will have even killed ourselves off. I'm depressed now, I am going to go crawl into a dark corner and cry myself to sleep.

However you slice it, interstellar travel requires a whole host of technologies we haven't developed and are not exactly on the horizon.

Cyrogenic freezing? Works if you want to kill someone, so far.

Huge, sustained accelleration? Don't see it.

And then there's the practical engineering problems. There's no repair shops from very early on. A heck of a lot of stuff has to work right for a long time in a self-contained package, with or without freezing the passengers.

IIRC, we couldn't even build small self-sustaining biospheres on earth that lasted for a year or so. What makes this any different or easier?

And, even with exercise, people's musculature falls off while in space. Not clear that we can get people somewhere interesting and have them able to enter the gravity well of the new joint.

Any craft with our current technologies looks like a one way trip in a very fancy and expensive coffin to me.

We can build a self-sustaining biosphere. There's no scientific or technological reason why not with today's knowledge. We haven't pulled it off yet, but that's mostly through lack of trying.

We can thus build a self-sustaining biosphere in space. It'd be ferociously expensive, but there's no scientific or technological objection.

What we can't do is move it anywhere.

We use rockets, giant explosions in a tube. They're inefficient, clumsy and unreliable, but great for giving a very high impulse for a very short time. Rockets, of course, use chemical reactions which are a few orders of magnitude less energetic than nuclear reactions. Rockets aren't going to get us anywhere because they plain don't scale well. Adding 1kg of mass to a rocket's payload means adding 40kg of fuel and fuel tank (for the Ariane V design anyway). As the mass to be lofted gets greater, a rocket looks worse and worse.

Part of a rocket's sucktitude comes from its chemical nature, chemical reactions are not very energetic compared to the mass of reactants used.

This brings us back to the biosphere. They're *heavy*. You're not pushing one anywhere with a rocket. Maybe not even with nuclear propulsion and I don't see macroscopic amounts of antimatter anywhere.

Larry, the "musclature falls off while in space" isn't true. It is if you define "space" as "microgravity" but there's no reason to make that definition, a vessel large enough for interstellar transport will have no problem spinning to assert an outward acceleration.

We can build a self-sustaining biosphere. There's no scientific or technological reason why not with today's knowledge.

We "should" be able to do this. IIRC, the two attempts we tried for limited space, enclosed ecosystems did not last nearly long enough to support non-suspended animation-type free flight.

There's a lot of things we "should" be able to do, but until we actually do them, maybe there's a few unobvious problems in the way. For all we know, there's a practical limit on "miniaturization" here that we today know nothing about. That would be extremely relevant for space flight. To the extent I've paid attention, our current experiments seem to me to fall rather short. It think we at least half agree on this.

And then, as you point out, moving it would be an issue. Which tends to imply some sort of miniaturization requirement.

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a vessel large enough for interstellar transport will have no problem spinning to assert an outward acceleration.

You may be right there. On the other hand, there's some practical questions. If the people flying the ship aren't in suspended animation, they would have to be very disciplined about their exercise program by all the sci fi and, from what I know about, long term space-inhabiting astronauts. How long to Alpha Centauri again?

Maybe that particular circle could be squared, but as you argue it, there is this looming trade-off between something we could credibly propel by any known technology compared to something big enough for biological beings to live in and live in well enough to enter someone else's gravity well on the other side.

Me, I was thinking about a much more tractably small ecosystem which, as far as I know, we don't yet have and may not be able to get in order to make the craft something plausibly small.

Realistic time frames. To mars: 2-3 months. To Jupiter: 3-4 years. 42 years seems like a tremendously short time, to the point that I'm skeptical of even that span. Of course, the proposed nearly magical ship travels with an external energy source. A more standard ship with an internal fuel storage would take far longer. The gulf of 60-100 years between the stars would be a distance not entirely uncrossable. It just won't be the same as crossing the globe.

5 BREAK OUT OF THE SOLAR SYSTEMTwo years ago the venerable Voyager space probes went through a funding scare. NASA, desperate for money, said it might have to shut them down. The ensuing public outcry kept them going. Nothing that human hands ever touched has gone as far as Voyager 1: as of press date, 103 astronomical units (AU)—that is, 103 times as far from the sun as Earth is—and picking up another 3.6 AU every year. In 2002 or 2004 (scientists disagree), it entered the mysterious multilayered boundary of the solar system, where outgoing solar particles and inflowing interstellar gases go mano a mano.

But Voyager was designed to study the outer planets, not interstellar space, and its plutonium batteries are running down. NASA has long had a mind to dispatch a dedicated probe, and an NRC report on solar physics argued in 2004 that the agency should start working toward that goal.

The spacecraft would measure the abundance of amino acids in interstellar particles to see how much of the solar system’s complex organics came from beyond; look for antimatter particles that might have originated in miniature black holes or dark matter; figure out how the boundary screens out material, including cosmic rays, which may affect Earth’s climate; and see whether nearby interstellar space has a magnetic field, which might play a crucial role in star formation. The probe could act as a miniature space telescope, making cosmological observations unhindered by the solar system’s dust. It might investigate the so-called Pioneer anomaly—an unexplained force acting on two other distant spacecraft, Pioneer 10 and 11—and pinpoint where the sun’s gravity brings distant light rays to a sharp focus, as a test of Einstein’s general theory of relativity. For good measure, scientists could aim the probe for a nearby star such as Epsilon Eridani, although it would take tens of thousands of years to get there.

Getting the thing hundreds of AU out within the lifetime of a researcher (and of a plutonium power source) would mean boosting it to a speed of 15 AU a year. The options boil down to large, medium and small—propelled, respectively, by an ion drive powered by a nuclear reactor, an ion drive powered by plutonium generators, or a solar sail.

The large (36,000-kilogram) and medium (1,000-kilogram) missions were honed in 2005 by teams led, respectively, by Thomas Zurbuchen of the University of Michigan at Ann Arbor and by Ralph McNutt of the Johns Hopkins University Applied Physics Laboratory. The small option seems the most likely to fly. ESA’s Cosmic Vision program is now considering a proposal from an international team of scientists led by Robert Wimmer-Schweingruber of the University of Kiel in Germany. NASA might join in, too.

A solar sail 200 meters across could carry a 500-kilogram spacecraft. After launch from Earth, it would first swoop toward the sun, going as close as it dared—just inside Mercury’s orbit—to get flung out by the intense sunlight. Like a windsurfer, the spacecraft would steer by leaning to one side or the other. Just before passing Jupiter’s orbit, it would cast off the sail and glide outward. To get ready, engineers need to design a sufficiently lightweight sail and test it on less ambitious missions first.

“Such a mission, be it ESA- or NASA-led, is the next logical step in our exploration of space,” Wimmer-Schweingruber says. “After all, there is more to space than exploring our very, very local neighborhood.” The estimated price tag is about $2 billion, including three decades’ operating expenses. Studying the other planets has helped humans figure out how Earth plugs into a grander scheme, and studying our interstellar environs would do the same for the solar system at large.

Originally posted by htom:Possible, through sufficently advanced technology that would appear to us as magic.

Has any other author, nevermind sci-fi author, had a quote that has been used more times?

Probably not. Back in the 1960's I wrote a thesis paper on the proposition that the Aladdin's Lamp Genie was actually a wish-granting high-tech wizard who appeared to many different cultures at about the same time.

Posted by Larry:You may be right there. On the other hand, there's some practical questions. If the people flying the ship aren't in suspended animation, they would have to be very disciplined about their exercise program by all the sci fi and, from what I know about, long term space-inhabiting astronauts. How long to Alpha Centauri again?

The best comparable psychology and physiology is that of prisoners. They don't appear to suffer any physical retardation due to their incarceration though mental effects can be variable. Where's the exercise program come from? With a constant 1g, there's no requirement for it, no more than there is on Earth.

The Mini-Mag system uses a magnetic field in order to trigger an explosion of compressed material in the form of small pellets weighing several grams. This explosion, although being significantly weaker than a nuclear explosion, creates plasma that is directed through a magnetic nozzle to generate vehicle thrust. The proposed technology enables the production of thrust at high efficiency, hopefully allowing drastic reduction of interplanetary travel time. According to calculations performed by AS&T, this type of propulsion system can produce the same thrust as the Space Shuttle Main Engine, with 50 times more efficiency.

Due to the magnetic compression thrust technology, spacecraft could be smaller and lighter. The spacecraft itself will only have to carry a relatively small amount of fissionable material as fuel and will be able to reach speeds of approximately 10% of the speed of light.

Every couple years, I post a poll in the Lounge regarding multi-generational starships, asking how many people would be willing to volunteer for such a mission even if they knew they would never reach the destination in their lifetimes. I'm always astonished how many people say they'd be willing to go. If Ars is at all representative of the population at large, then even if we decided that most of those volunteers were physically or psychologically unsuited to the mission, I think we'd still have plenty of willing colonists.

However, I don't think we need to worry too much about the difficulties of interstellar travel right now. It's enough to know that it's possible, if inconvenient. But it's a problem for some far-off future generation. The best thing we could do to speed its coming is to focus on goals that are within reach now, like accelerating the exploration and colonization of the middle solar system.

an awful lot of the speculation here gets basic reaction physics and mass laws wrong, and confuses all sorts of magnitudes of things.....

Assuming that we don't obtain some "warp drive" which frees us from general relativity (and I happen to think that is a foregone conclusion) then all realistic means of near-interstellar travel will be "slow" compared to the speed of light.

We can live with that, if we are patient. Obviously, interstellar travel/colonization will primarily depend on "learning to be patient" ... and strategies will end up being a tradeoff of vehicle mass vs speed vs cost -- just like other vehicle design problems.

In theory fusion schemes might get high-mass-ratio vehicles up to 5-10% of C, but the mass ratios look ugly, and right now the required magnetic confinement looks very heavy and very large. The nasty thing about the fusion schemes are that the easiest fusion scheme, deuterium/tritium (or equivalently Li6 to breed tritium, ala current nuclear weapons) -- is very dirty in terms of neutron flux. Right now D/T is the only thing we are close to being able to drive, either inertially or with magnetic confinement. In order to build a reaction system (i,e, "rocket") we must use magnetic confinement, but the system could be an inertial/magnetic hybrid.

D/D is better than D/T in terms of total neutron flux, but not by even an order of magnitude. The reason is simple: the D + D -> He4 reaction can't go directly without some third body to carry off enery; and the three-body cross-section is incredibly small at the densities we can possibly get.

Instead what you get are chain reactions where

D + D -> T + p

or

D + D -> He3 + n

and then

T + D -> He4 + n

and

He3 + D -> He4 + p

and a bunch of other reactions from the stew. But the end result is that D/D reactions don't cleanly produce He4, instead there is a lot of neutron (and energetic proton) production carrying off most of the energy. You can't magnetically confine those neutrons.

Unless you can get an enormous reaction zone opacity for neutrons, so that the neutrons thermalize in the plasma (as they do in a star of course) ... you've got a problem.

There's an article on the Wikipedia about "aneutronic fusion" which is relevant and folks should read:

It mostly dumps a big wet blanket on all the reaction schemes which put out less than about 10% of their energy in neutrons. The two schemes which are vaguely possible are He3+He3, and p+B11 (Boron11). Both depend on isotypes which are not easy to obtain. Years back there was a paper in JBIS about a very speculative scheme for interplanetary probes which would use p+B11 fusion, ... this was waaaay speculative and did major handwaving about how they'd make the reaction work, and ignored most of the issues discussed in that wikipedia article. Mostly I take that article as sci-fi they managed to sneak into a sort-of scientific journal... and I think that as far as we can see ... all "realistic" schemes will live with a lot of neutron flux.

Magnetically-confined fission schemes (potentially deuterium/tritium boosted -- basically hybrid fission/fusion) look much simpler and much closer to something we can "make work with not too much more than what we know" ... and they can yield a decent specific impulse ... but they are dirty, dirty, dirty in terms of neutron flux too.

But my point is that AFAIS, you are stuck with a nasty neutron flux problem no matter what you do (I think) so you might as well make it easy on yourself, and go with fission/fusion schemes. The perverse point of this analysis is that actually "the engines aren't so hard." We sure know how to run a fission reaction in a magnetically confined axial plasma with (intentionally) poor containment time.

Shielding the vehicle from this is "not so bad" ... you just need the engine to be far from the vehicle, and have some shielding on the vehicle. We'll need substantial shielding on the vehicle anyway, to deal with cosmic rays.

The bigger problem is the materials problem of neutron damage to the engine and its magnet coils, superconducting certainly. Let me "wave my fingers airly" to suggest that these problems aren't insuperable ... given what the Tokamak boys like JSTOR have tested and expect to do, etc.

But the point of this discussion is that all of these engine schemes need scale... at any technology point there is a minimum size you can build which will sustain "ignition" ... and that size looks BIG, and probably massive. And if you want to thermalize those neutrons in the plasma itself ... that problem requires that the product of the plasma density * length scale be large, and that means scale (high density means very powerful magnets which is a scale issue too)

And so these suckers aren't going to be small. And probably they are going to go considerably less than 0.01 c once you work it up, so we are talking 1000's of years to get to plausible stars.

But once you admit this, then the scale of these spacecraft gets big enough that the idea of a small community in transit doesn't seem impossible. Potentially cramped and stressful yes, impossible no. People are more adaptive than you might think, but that society might be pretty weird by our current standards.

Can we do this? Yes, I think we can. WILL we do this ... damn hard to say. Obviously not cheap, not easy, and probably pretty risky too. But we can consider a variety of strategies/developments which might be possible and would make it more practical:

One obvious thing is that "the crew" it might be all female -- just carrying frozen sperm. That automatically stops inbreeding and solves a set of social problems, and halves the number of people needed to keep a minimum reproductive capacity (needed to reduce the probability that the generations stop due to the bad luck of having a very small number of potential mothers, all of whom become infertile etc).

* we know that some invertebrates can be frozen solid, and revived. They produce compounds which protect cell walls against ice-particle rupture. Possibly we could engineer "humans" to do this. Possibly this could be aided by using a very careful temperature/pressure regime -- at very high pressures ice has other phases and higher densities.

* Or just go whole-hog ... we freeze sperm and embryos right now. Works great. Artifical wombs really don't look that tough. We just need to figure out how to raise the babies, into something functional. Will AI+robots be good enough? How good do you need?

* what is "human?" How far outside the box are you willing to think? At what point of AI do we concede that they are "human" ... ethically ... even though they might be wildly different from us?

Anyway, if humanity has another 10,000 years of technological society .. (which I don't take for granted given what we are screwing up) ... I think a lot of the problems look pretty solvable. And obviously they will look a lot more solvable if we have colonized anything else in our solar system (the moon and mars for starters).

I think the far bigger challenge is .... will we ever get outside our galaxy? That's a tough one. Those transit times look scary.

But even that doesn't look hopeless to me. I'd point out too things -- one low tech and one high:

* galaxies eject stars, and they have fairly high velocities. The mechanics and trajectories are very predictable. Hop a ride with a star going toward a galaxy you want to.

* There is an interesting trajectory which uses a black hole: if you take a hyperbolic entry trajectory which will pass close to the event horizon, and then fire a "rocket" near the peri-hole to accelerate yourself ... the interesting thing is that your exit velocity is huge because you have effectively "converted" much of the mass into energy at close to mc2 due to the potential depth (and local velocity) in the gravitational well.

In principle you can get exit velocities close to the speed of light this way. What you need is a really, really big, and quiet, black hole. Most galaxies have that black hole, whether it is quiet enough is TBD.

Of course just about the only way you slow down is to do the reverse. You've got to dive close to a big black hole at your destination.

48 J per second * Number of seconds in a year times 2 / c^2 = 33 micrograms of mass.

So, for every kilogram of mass you want to transport, you need to bring with you 33 micrograms of fuel. Not too shabby.

One other minor correction:

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It mostly dumps a big wet blanket on all the reaction schemes which put out less than about 10% of their energy in neutrons. The two schemes which are vaguely possible are He3+He3, and p+B11 (Boron11). Both depend on isotypes which are not easy to obtain.

B-11 is 80% of naturally occurring Boron, which is a fairly common element on Earth.

Assuming that we don't obtain some "warp drive" which frees us from general relativity (and I happen to think that is a foregone conclusion) then all realistic means of near-interstellar travel will be "slow" compared to the speed of light.

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I think the far bigger challenge is .... will we ever get outside our galaxy? That's a tough one. Those transit times look scary.

Sorry, BadAndy, but these two quotes do not compute. (Unless the top quote is meant to mean the opposite of what I'm taking it to mean here, it's not entirely clear if you believe not beating GR is inevitable or if beating GR is inevitable)

If beating c is a foregone conclusion, then beating "warp drive" is also a foregone conclusion, then beating "transwarp drive" is also a foregone conclusion, then beating "hyperwarp drive" is also a foregone conclusion. The end result is that we can travel at infinite velocity and get wherever the hell we like in zero time.

Adding time does not a solution make. We still can't fly by flapping our arms, yet this has been a staple of 'science fiction' for thousands of years.

Originally posted by Hat Monster:Sorry, BadAndy, but these two quotes do not compute. (Unless the top quote is meant to mean the opposite of what I'm taking it to mean here, it's not entirely clear if you believe not beating GR is inevitable or if beating GR is inevitable)

My apologies for ambiguous prose. I meant that I see no chance for any exemption from general relativity big enough and available at low enough energies that we could have some sort of "warp drive."

Interestingly there are theoretical solutions to Einstein's equations which allow a local domain at > c, but these involve utterly enormous masses & energies moving in exact ways nearby, which look impossible to create/sustain. The not-too-surprising effect/consequence of these bizarre time-space geometries is that the far-field is "shielded" from the anomaly. But to sustain this the energies and masses grow with time (I think exponentially, IIRC). There are pretty obvious reasons to come to the conclusion that we aren't going to get some sort of scot-free exemption: it breaks too much very-well tested physics. It also causes major problems with causality, although that gets to be a murky area.

A "hot" local exemption seems more possible at high enough energies, but does us no good. The inflationary epoch of our universe amounts to a hot exception to general relativity as we see it right now (we are living in a "cold universe" and general relativity is the "low" energy density asymptotic solution for some reality we don't completely understand yet), but something at higher energies than gluon plasma doesn't permit "us" to exist.

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Lord Frith: B-11 is 80% of naturally occurring Boron, which is a fairly common element on Earth.

I take your correction. I mis-remembered that JBIS article, and I don't have a copy. B + p is a not very attractive fusion reaction in any event -- see that Wikipedia article.

He3 is damn rare and hard to get. Mining out of the surface layer of the moon (which has been capturing the solar wind) or getting out of the atmospheres of the gas giants .... looks bad either way to me. And He3 + He3 has its problems.

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No, most is basing it around the ability to carry energy. Reaction mass is typically a different problem, and not so hard with an acceptable form of energy.

For example, matter/antimatter annhilation would create only photons, which would then be ejected to create propulsion. Fairly efficient method of propulsion.

How much would be needed? Well, If I want to maintain a local acceleration of 1g for about two years, for a 1kg mass we would need:

48 J per second * Number of seconds in a year times 2 / c^2 = 33 micrograms of mass.

So, for every kilogram of mass you want to transport, you need to bring with you 33 micrograms of fuel. Not too shabby.

Nope, doesn't work that way. You are just calculating delta-E. In effect your solution is the "infinite free reaction mass" solution. Also, for the time involved you are ignoring the relativistic effects.

Your 1 g for two years, calculated with Newtonian physics would yield roughly 6E8 m/s, or about twice the speed of light. That's not gonna happen.

Even more fundamental though is the momentum equation:

The thrust of a pure "photon drive", i.e. what you get from matter-antimatter simply produces the momentum of the resulting photons. That momentum is p = (h nu)/c = E/c.

There is one minor subtlety -- the photons are emitted isotropically and to get thrust we must "do something" .. if you imagine them emitted from a point source in a deep parabolic "mirror" then we get all of their momentum as thrust. Unfortunately those photons are at gamma wavelengths which we can't mirror ... but we can absorb. If we absorb all of them going forward then to first order we get 1/2 * 1/sqrt(3) of the total momentum -- as a practical matter that's as good as "pure photon drive" gets.

Another way of looking at the problem is the fundamental rocket equation for efficiency. The "most efficient" Newtonian rocket in terms of energy has the solution that the exit velocity == the vehicle velocity. This is intuitively obvious in a newtonian view because it has the result that the reaction mass has "no kinetic energy" in the non-accelerating reference frame. So obviously all of the energy is being transferred to the rocket.

Your photon drive only becomes energy efficient at vehicle velocities near c. At low velocities it is horribly inefficient, unless you have much more reaction mass to apply it to.

The one hope for really high performance "anti-matter" drives is to use the energy to drive a "Bussard ramjet" ... magnetically collect H, heat it with the anti-matter, and squirt it back out. This looks like a bitch, for many reasons.

(The original concept of a Bussard ramjet is hopeless because we can't run H+H+H+H fusion without the "containment product" of a star ... to run a carbon cycle.)

48 J per second * Number of seconds in a year times 2 / c^2 = 33 micrograms of mass.

So, for every kilogram of mass you want to transport, you need to bring with you 33 micrograms of fuel. Not too shabby.

Nope, doesn't work that way. You are just calculating delta-E. In effect your solution is the "infinite free reaction mass" solution. Also, for the time involved you are ignoring the relativistic effects.

Your 1 g for two years, calculated with Newtonian physics would yield roughly 6E8 m/s, or about twice the speed of light. That's not gonna happen.

Actually, 2 years since one is spent accelerating, one is spent de-accelerating.

Yes, you would approach speeds very close to the speed of light doing this -- arbitrarily close, in only one year. However, the time required from your initial rest frame would be very, very, long. And if you accelerated long enough to get to light speed... Maybe we need another thread on this.

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Even more fundamental though is the momentum equation:

The thrust of a pure "photon drive", i.e. what you get from matter-antimatter simply produces the momentum of the resulting photons. That momentum is p = (h nu)/c = E/c.

There is one minor subtlety -- the photons are emitted isotropically and to get thrust we must "do something"

So I now have a way to make energy. Use it to power a laser. Aim laser out back of ship. Done. (Less efficient, but still, a lot less required mass.)

48 J per second * Number of seconds in a year times 2 / c^2 = 33 micrograms of mass.

So, for every kilogram of mass you want to transport, you need to bring with you 33 micrograms of fuel. Not too shabby.

BadAndy: Nope, doesn't work that way. You are just calculating delta-E. In effect your solution is the "infinite free reaction mass" solution. Also, for the time involved you are ignoring the relativistic effects.

Your 1 g for two years, calculated with Newtonian physics would yield roughly 6E8 m/s, or about twice the speed of light. That's not gonna happen.

Lord Frith: Actually, 2 years since one is spent accelerating, one is spent de-accelerating.

Yes, you would approach speeds very close to the speed of light doing this -- arbitrarily close, in only one year. However, the time required from your initial rest frame would be very, very, long. And if you accelerated long enough to get to light speed... Maybe we need another thread on this.

NO. You are not right at all, and you wouldn't get anywhere near the speed of light at all, just work out the momentum equation:

Because this is so slow (assuming you are starting from zero) no relativistic corrections are needed, either.

That is effing pathetic ... 9 m/s = 32 km/h ... you wouldn't get a speeding ticket in most grade-school speed zones.

I tried to say it nicely the first time. Apparently I was too subtle. You cannot beat the momentum equation, not with anything which is a rocket anyway.

Your "energy balance" claims have nothing to do with getting to very high velocities, per se. If they were right they ought to work at any velocity. And if you were right then every rocket is idiot and every rocket engineer is the idiot who engineered it.

To make this clear, very roughly, 10 km/sec (1e4 m/s) is planetary escape velocity. E = 1/2 m v^2, so we calculate the corresponding energy per kg to be 0.5e8 joules.

The combustion of hydrogen with oxygen yields 141.6 MJ/kg (of hydrogen) = 1.4e8 J/kg ... so roughly speaking we would need 1/3 kg of hydrogen to get enough energy to get to planetary escape velocity ... if you were right.

Even if we note that we will need 8 kg of oxygen for every kg of hydrogen ... if you were right we'd all be tooling around space in our own little home-built space buggies, right now.

Incidentally, if we could make a mach 30 scramjet work ... that energy analysis is "only wrong by roughly an order of magnitude" ... and that's why folks are seriously pursuing high-mach-number scramjets. The whole point is that they use atmospheric reaction mass, so the momentum equation is far more favorable.

And if you can make a "space elevator" then the energy analysis is bang on ... because now you are using the earth as your reaction mass. And that's why folks fantasize about space elevators from earth.

I think that space elevators from earth are far-out sci fi. But tether-coupled systems in space have real possibilities ... albeit a lot less fantastic (in both senses) than a "space elevator" all the way.

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So I now have a way to make energy. Use it to power a laser. Aim laser out back of ship. Done. (Less efficient, but still, a lot less required mass.)